Sunday, February 2, 2014

Interpret the 3-lead
rhythm strip shown in Figure-1
— obtained from a patient in a fairly (but
not completely) regular SVT rhythm.

What is the reason for the slight
change in QRS morphology from beat-to-beat?

Figure-1: 3-lead rhythm strip — obtained from a patient in a regular SVT rhythm. What is the reason for the slight change in QRS morphology from beat-to-beat? (Figure reproduced from ECG-2014-ePub). NOTE — Enlarge by clicking on Figures — Right-Click to open in a separate window.

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Answer to Figure-1: Although one might at first be tempted
to interpret the rhythm as a form of bigeminy — a more accurate interpretation
would be electricalalternans.
While we would accept general description of this rhythm as representing a “regular” SVT (SupraVentricular Tachycardia) — there is in
fact slight-but-real phasic variation
in the R‑R interval occurring every-other-beat. This is not due to a form of
bigeminy — but rather to R‑Ralternans. In addition — there is both QRS
alternans (red and blue double arrows
in Figure-2) — andT wave alternans (red and blue circlesin Figure-2). That is — QRS
morphology changes every-other-beat.
This is subtle in lead V1 — but more noticeable in lead V2 where the initial R
wave manifests an obvious difference in height from one beat to the next.
Similarly — T wave morphology changes every-other beat, with this clearly more
noticeable in lead V2 which manifests extra peaking of every-other-T wave (red and blue circles in lead V2).

Clinical implications of these forms of
electrical alternans in a patient with SVT — are that reentry is almost certain
to be involved in the mechanism. There may or may not be a concealed accessory
pathway.

Figure-2: We have labeled the 3-lead rhythm strip recorded in Figure-1. There is slight shortening of the R-R interval every-other beat = R‑Ralternans. In addition — there is both QRS alternans (red and blue double arrows) — andT wave alternans (red and blue circlesin Figure-2). That is — QRS and T wave morphology changes every-other-beat. (Figure reproduced from ECG-2014-ePub).

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What
is ElectricalAlternans?

The fascinating phenomenon of electricalalternans — is a relatively uncommon
clinical entity that is frequently misunderstood. It is often overlooked when it does occur. A look at Figure-1
explains why: This ECG sign can be subtle indeed.

Electrical alternans is a general term
that encompasses a number of different
pathophysiologic mechanisms. Its occurrence is not limited to pericardial tamponade — but instead has been
associated with an expanding array of clinical conditions.

Distinction should be made between electricalandmechanical alternans. The term “alternans”
itself — merely indicates that there is phasicfluctuation in somecardiacsignal from one
beat to the next within the cardiac cycle. This may be in the strength of the
pulse (or the blood pressure recorded)
— or it may be in one or more waveforms in the ECG recording.

NOTE: It may be helpful to first define other alternans phenomena that may
sometimes be confused with the various ECG manifestations (especially since these other forms of alternans phenomena may also be
seen with cardiac tamponade).

Pulsusalternans
— is a mechanical form of alternans.
The rhythm is regular — but
cardiac output varies from beat-to-beat. It is seen with severe systolic
dysfunction. Pulsus alternans should be distinguished from a bigeminalpulse — in which a weaker beat follows the stronger beat by
a shorter time interval (as occurs when the alternating beat is a
PVC, which understandably generates less cardiac output).

Pulsus alternans should also be
distinguished from pulsusparadoxus —
in which there is a palpable decrease in pulse amplitude (or a measured drop of >10mm in blood pressure) during quiet
inspiration. While pulsus alternansandparadoxus may both be seen with
pericardial tamponade — they are different
phenomena than the various types of electrical
alternans.

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ElectricalAlternans:Definition/Features/Mechanisms

Electrical alternans — is a beat-to-beat variation in any one or more parts of the ECG
recording. It may occur with every-other-beat
— or with some other recurring ratio (3:1; 4:1; etc.). Amplitude or direction of the P wave, QRS complex,
ST segment and/or T wave may all be
affected (although P wave alternans is
rare). Alternating interval duration (of
PR, QRS or QT intervals) may also be seen.

Electricalalternans
— was first observed in the laboratory by Herring in 1909. It was reported
clinically by Sir Thomas Lewis a year later, who characterized the phenomena as
occurring, “either when the heart muscle
is normal but the heart rate is very fastorwhen there is serious heart disease and the rate is normal”. This
1910 description by Lewis serves well to this day to remind us of the 2
principal clinical situations in which electrical alternans is most often
encountered: i) Supraventricular reentry tachycardias; andii) Pericardial tamponade.

Mechanisms:There are 3 basic
types of electrical alternans phenomena — each relating to a different pathophysiologic
mechanism: i)Repolarization alternans; ii)
Conduction and Refractoriness alternans; andiii) Alternans due to abnormal cardiac
motion. A common cellular mechanism may underlie each of these processes
relating to abnormal calcium release or reuptake within the sarcoplasmic
reticulum.

Repolarizationalternans
— entails beat-to-beat variation in
the ST segmentand/orT wave.
Alternation in ST segment appearance (or
in the amount of ST elevation or depression) — is often linked to ischemia.
In contrast — T wave alternation is more often associated with changes in heart
rate or in QT duration (especially when
the QT is prolonged). In patients with a long QT — T wave alternans may
forebode impending Torsades de Pointes. Both ST segment and T wave alternans
have been known to precede malignant ventricular arrhythmias. Thus, this type
of electrical alternans may convey important adverse prognostic implications
when seen in certain situations. That said — a variety of clinical conditions have been associated with repolarizationalternans, such that adverse
prognostic implications do not always follow. Among these clinical conditions
are congenital long QT syndrome — severe electrolyte disturbance (hypocalcemia; hypokalemia; hypomagnesemia)
— alcoholic or hypertrophic cardiomyopathy — acute pulmonary embolus —
subarachnoid hemorrhage — cardiac arrest and the post-resuscitation period —
and various forms of ischemia (spontaneous
or induced by treadmill testing or other stimulus).

Conduction and
Refractorinessalternans — entails variance of impulse
propagation along some part of the
conduction system. This may result from fluctuations in heart rate or in
nervous system activity or from pharmacologic treatment. ECG manifestations
from this form of alternans may include alternatingappearance of the P wave,
QRS complexor alternating
difference in P-R or R-R interval duration. In particular —
QRS alternans during narrow SVT
rhythms has been associated with reentry tachycardias. While
identification of QRS alternans
during a regular SVT often indicates retrograde conduction over an AP (Accessory Pathway) — this
phenomenon has also been seen in patients with simple PSVT/AVNRT that
exclusively limits its reentry pathway to the AV Node. Therefore —
identification of QRS alternans during a regular
SVT does not prove the
existence of an accessory pathway. Conduction
and refractoriness alternans may be
seen with WPW-related as well as AV Nodal-dependent reentry tachycardias
— atrial fibrillation — acute pulmonary embolus — myocardial contusion — and
severe LV dysfunction.

Cardiac Motionalternans
— is the result of cardiac movement rather than electrical alternation. The
most important clinical entity associated with motionalternans is large pericardial effusion
— though motion alternans has also been observed in some cases of hypertrophic
cardiomyopathy. It is important to appreciate that not all pericardial effusions produce electrical alternans.
Development of total electrical alternans (of
P wave, QRS complex and T wave) — is likely to be a harbinger of impendingtamponade. Unfortunately — the
sensitivity of total electrical
alternans is poor for predicting tamponade (ie, most patients who develop tamponade do not manifest preceding
electrical alternans). Therefore — it may be helpful if you see total
electrical alternans in a patient with a large pericardial effusion — but
failure to see this ECG sign in no way
rules out the possibility that tamponade is occurring. Echo studies in patients
with documented cardiac tamponade confirm that electrical alternans is
synchronous with and a direct result of the pendulous movement of the heart within the enlarged, fluid-filled pericardial sac of a
patient with large pericardial effusion.

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ElectricalAlternans:KEY Clinical Points

In summary, electrical alternans is not common — but it does
occur. You will see it. You
have probably already seen it a
number of times without even
realizing it. Electricalalternans
is a fascinating but advanced
concept.

In our experience — electricalalternans is most often seen in
association with regular SVT rhythms (as seen inFigure-1). Seeing it in this context suggests (but does not prove) the existence of an
AP (Accessory Pathway).
Regardless of whether the mechanism of the regular
SVT is AVNRT or AVRT — it is likely that reentry is involved. This conclusion
may prove useful in contemplating potential investigative and therapeutic
interventions.

In a patient with pericarditis — a
large heart on chest X-ray — or simply unexplained dyspnea — recognition of
electrical alternans should suggest the possibility of a significant pericardialeffusion that may be associated with
tamponade. That said — electrical alternans is a nonspecificECG sign that may also indicate
myocardial ischemia, LV dysfunction and/or
possibility of any of a number of other
precipitating factors. BOTTOM Line: If you
see electrical alternans — Look for an underlying
clinical condition that may be responsible for this ECG sign.

Development of electricalalternans per se — conveys no adverse prognostic implications beyond those associated with severity of
the underlying disorder. Two
exceptions to this general rule are: i)
In a patient with QT prolongation or severe ischemia — recognition of
electrical alternans may portend deterioration
to Torsades or VT/VFib; andii)
In a patient with a large pericardial effusion — development of total
electrical alternans (of P wave, QRS
complex and T wave) suggests there may now be tamponade.

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ACKNOWLEDGMENT: My appreciation to Jenda Enis Stros
for allowing me to use the ECG in Figure‑1.

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- For more information — GO TO:

For a pdf of Section 14.0 — on ElectricalAlternans (from our ECG-2014-ePub).